553Third gear

At even higher road speeds in D, drive position, the governor pressure will have risen to a point where it is able to overcome the spring and throttle pressure load of the 2-3 shift valve. This causes the spool valve to shift over so that the line pressure passage feed from the manual valve is uncovered. Line pressure will now flow through the 2-3 shift valve where it is directed to the high and reverse clutch to energize the respective fixed and rotating friction plates. At the same time, line pressure passes to the second gear band servo on the release side to disengage the band. Consequently both overdrive and forward planetary gear sets lock-up, permitting the input drive from the torque converter to be transmitted directly through to the transmission's output shaft.

The actual vehicle speed at which the 2-3 shift valve switches over will be influenced by the throttle opening (throttle pressure). A low throttle pressure will cause an early gear upshift whereas a large engine load (high throttle pressure) will raise the upshift speed.

Fig. 5.6 Hydraulic control system (D) range first gear

With still higher road speeds in D, drive position, the increased governor pressure will actuate the 3-4 shift valve, forcing it to shift across so that it covers up the line pressure supply passage and at the same time uncovers the exhaust or drain port. As a result, the line pressure exhausts from the release side of the overdrive band servo which then permits the band to be energized. At the same time the drive clutch will be de-energized because of the collapse of line pressure as it is released through the 3-4 shift valve exhaust port.

Fig. 5.7 Hydraulic control system (D) range second gear

Under these operating conditions the overdrive shaft planetary gear set reduces the intermediate shift speed and, since the forward clutch is in a state of lock-up only, this speed step up is transmitted through to the output shaft.

5.5.5 Reverse gear (Fig. 5.10) With the manual valve in R, reverse position, line pressure from the manual valve is directed via the 2-3 shift valve to the release side of the second gear band servo, causing the band to disengage. At the

Fig. 5.8 Hydraulic control system (D) range third gear

same time line pressure from the same supply passage engages the high and reverse clutch. The manual valve also supplies line pressure to the low and reverse band brake via the 1-2 shift valve to hold the reverse planetary carrier. In addition, line pressure from the pressure regulating valve output side is directed via the 3-4 shift valve to the release side of the overdrive brake servo to disengage the band and to the drive clutch piston to engage the friction plates. Note that both band brake servos on the applied sides have been exhausted of line pressure and so has the forward clutch piston chamber.

Fig. 5.9 Hydraulic control system (D) range fourth gear

5.5.6 Lock-up torque converter (Fig. 5.11)

Introduction To overcome the inherent relative slip which always occurs between the torque converter's pump impeller and the turbine runner, even driving at moderate speeds under light load conditions, a lock-up friction clutch may be incorporated between the input pump impeller and the turbine output shaft. The benefits of this lock-up can only be realised if the torque converter is allowed to operate when light torque demands are made on the engine and only when the converter is operating above its torque multiplication range that is beyond the coupling point. Consequently, converter

lock-up is only permitted to be implemented when the transmission is in either third or fourth gear. The advantages of bypassing the power transfer through the circulating fluid and instead transmitting the engine's output directly to the transmission input shaft eliminates drive slippage, thereby increasing the power actually propelling the vehicle. Due to this net gain in power output, fuel wastage will be reduced.

Lock-up clutch description The lock-up clutch consists of a sliding drive plate which performs two functions; firstly to provide the friction coupling device and secondly to act as a hydraulic con

Fig. 5.11 (a and b) Lock-up torque converter

(a) Torque converter oclt-up disengaged

Fig. 5.11 (a and b) Lock-up torque converter trolled piston to energize or de-energize the clutch engagement facings. The lock-up drive plate/piston is supported by the turbine hub which is itself mounted on the transmission input shaft. A transmission damper device, similar to that used on a conventional clutch drive-plate, is incorporated in the lock-up plate to absorb and damp shock impacts when the lock-up clutch engages.

Lock-up control The automatic operation of the converter lock-up is controlled by a speed cut valve and a lock-up control valve. The function of these valves is to open and close fluid passages which supply and discharge fluid from the space formed between the torque converter casing and the lockup drive plate/piston.

Lock-up disengaged (Fig. 5.11(a)) With the vehicle driven in either first or second gear at relatively low speeds, low governor pressure permits the speed cut and lock-up control valve return springs to push their respective plunger to the right. Under these conditions, pressurized fluid from the torque converter flows into the space separating the lockup plate/piston from the turbine. At the same time, fluid from the oil pump is conveyed to the space formed between the torque converter's casing and the lock-up plate/piston via the lock-up control valve and the central axial passage in the turbine input shaft. Consequently, the pressure on both sides of the lock-up plate will be equalized and so the lock-up plate/piston cannot exert an engagement load to energize the friction contact faces.

Lock-up engaged (Fig. 5.11(b)) As the speed of the vehicle rises, increased governor pressure will force the speed cut valve plunger against its spring until it uncovers the line pressure passage leading into the right hand end of the lock-up control valve. Line pressure fed from the high and reverse clutch is directed via the speed cut valve to the right hand end of lock-up control valve, thereby pushing its plunger to the left to uncover the lock-up clutch drain port. Instantly, pressurized fluid from the chamber created between the torque converter casing and lock-up plate/piston escapes via the central input shaft passage through the wasted region of the lock-up control valve plunger back to the inlet side of the oil pump. As a result, the difference of pressure across the two sides of the lock-up plate/piston causes it to slide towards the torque converter casing until the friction faces contact. This closes the exit for the converter fluid so that full converter fluid pressure is exerted against the lock-up plate/piston. Hence the input and output shafts are now locked together and therefore rotate as one.

Speed cut valve function The purpose of the speed cut valve is to prevent fluid draining from the space formed between the converter casing and lock-up plate/piston via the lock-up control valve if there is a high governor pressure but the transmission has not yet changed to third or fourth gear. Under these conditions, there is no line pressure in the high and reverse clutch circuit which is controlled by the shift valve. Therefore when the speed cut valve plunger moves to the left there is no line pressure to actuate the lock-up control valve so that the lock-up plate/ piston remains pressurized on both sides in the disengaged position.

Do It Yourself Car Diagnosis

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